Terephthalic acid is commercially produced by oxidation of paraxylene in the presence of a catalyst, such as, for example, Co, Mn, Br and a solvent. Terephthalic acid used in the production of polyester fibers, films, and resins must be further treated to remove impurities present due to the oxidation of para-xylene. Typical commercial process produce a crude terephthalic acid then dissolve the solid crude terephthalic acid in water at high temperatures and pressures, hydrogenate the resultant solution, cool and crystallize the terephthalic acid product out of solution, and separate the solid terephthalic product from the liquid as discussed in U.S. Pat. No. 3,584,039 herein incorporated by reference.
A number of processes for producing the purified terephthalic acid solid have been developed and are commercially available. Usually, the purified terephthalic acid solid is produced in a multi-step process wherein a crude terephthalic acid is produced. The crude terephthalic acid does not have sufficient quality for direct use as starting material in commercial polyethylene terephthalate(PET). Instead, the crude terephthalic acid is usually refined to purified terephthalic acid solid.
Liquid phase oxidation of p-xylene produces crude terephthalic acid. The crude terephthalic acid is dissolved in water and hydrogenated for the purpose of converting 4-carboxybenzaldehyde to p-toluic acid, which is a more water-soluble derivative, and for the purpose of converting characteristically yellow compounds to colorless derivatives. Significant 4-carboxybenzaldehyde and p-toluic acid in the final purified terephthalic acid product is particularly detrimental to polymerization processes as they may act as chain terminators during the condensation reaction between terephthalic acid and ethylene glycol in the production of PET. Typical purified terephthalic acid contains on a weight basis less than 250 parts per million (ppm) 4-carboxybenzaldehyde and less than 150 ppm p-toluic acid.
The crude terephthalic acid typically contains on a weight basis from about 800 to 7,000 parts per million (ppm) 4-carboxybenzaldehyde and about 200 to 1,500 ppm p-toluic acid as the main impurities. The crude terephthalic acid also contains lesser amounts, about 20–200 ppm range, of aromatic compounds having the structures derived from benzil, fluorenone, and/or anthraquinone, which are characteristically yellow compounds as impurities resulting from coupling side reactions occurring during oxidation of p-xylene
Such a purification process typically comprises adding water to the crude terephthalic acid to form a crude terephthalic acid slurry, which is heated to dissolve the crude terephthalic acid. The crude terephthalic acid solution is then passed to a reactor zone in which the solution is contacted with hydrogen in the presence of a heterogeneous catalyst at temperatures of about 200° to about 375° C. This reduction step converts the various color causing compounds present in the crude terephthalic acid to colorless derivatives. The principal impurity, 4-carboxybenzaldehyde, is converted to p-toluic acid.
Typical crude terephthalic acid contains excessive amounts of both 4-carboxybenzaldehyde and p-toluic acid on a weight basis. Therefore, to achieve less than 250 ppmw 4-carboxybenzaldehyde and less than 150 ppmw p-toluic acid in the purified terephthalic acid requires mechanisms for purifying the crude terephthalic acid and removing the contaminants.
In many processes, colored impurities are hydrogenated to colorless derivatives and leave the process with the terephthalic acid solid product and waste water streams. However, one embodiment of this invention provides an attractive process to produce a purified carboxylic acid slurry by utilizing a solid-liquid displacement zone comprising a solid-liquid separator after oxidation of a crude carboxylic acid slurry product and prior to final filtration and drying without the use of an hydrogenation step.